Future multimedia services such as videophones require high quality sound at high playback levels. Because of the size constraint, small loudspeakers in handsets introduce both linear and nonlinear distortions into the sound, especially at high sound volumes. Predistortion or equalization is a general technique for compensating for distortions of small loudspeakers. The existing equalization techniques compensate for distortions at low playback levels suitable for voice communication; at high playback levels needed for multimedia communication, they introduce more distortion.
U.S. patent application Ser. No. 11/145,411, entitled “Method and Apparatus for Loudspeaker Equalization Using Adaptive Feedback and Exact Inverse”, (referred to hereinafter as “Lashkari”) describes a method and an apparatus for loudspeaker equalization using adaptive feedback and an exact inverse. As described in Lashkari, the exact inverse improves the perceptual quality at high playback levels and performs much better than the conventional methods described in prior arts. However, the method described in Lashkari is not guaranteed to produce a stable inverse. A suboptimal heuristic method for stabilizing the exact inverse was described in Lashkari, which makes the exact inverse a pseudo-exact inverse.
U.S. Pat. No. 5,068,903, entitled “Method and Arrangement for Linearizing the Frequency Response of a Loudspeaker”, issued Nov. 26, 1991 describes a method for equalizing the linear frequency response of the loudspeaker using impedance feedback. This patent describes equalizing the mechanical resonances arising from the loudspeaker, its enclosure or their combination. The impedance feedback is used to measure the loudspeaker's linear response and therefore no a priori measurement of the loudspeaker characteristics is necessary. Compensation of nonlinear distortion is not explicitly addressed in this patent.
U.S. Pat. No. 5,542,001, entitled “Smart Amplifier for Loudspeaker Motion Feedback Derived from Linearization of a Nonlinear Motion Responsive Signal”, issued Jul. 30, 1996 describes a method for compensating the nonlinear distortion of a loudspeaker by deriving a feedback signal based on the back electromotive force (emf) that is proportional to the cone motion. The method described in this patent tries to compensate for the nonlinearity of the force factor BLi as a function of the voice coil displacement. Therefore, the force factor vs. displacement curves must be measured for each loudspeaker. Furthermore, this method also requires loudspeaker-specific tuning of the analog circuitry to approximate the correction factor.
U.S. Pat. No. 5,600,718, entitled “Apparatus and Method for Adaptively Precompensating for Loudspeaker Distortions”, issued Feb. 4, 1997, uses the electrical equivalent circuit of an electrodynamic loudspeaker and output feedback to adjust the parameters of the precompensator over time. However, the loudspeaker model and the precompensator structure are completely different from the present invention. Also, the method uses an analog circuit to model the loudspeaker and fine tuning is required to match the circuit to the loudspeaker. A microphone is also needed to provide the feedback signal which is a drawback both in terms of the implementation and also because it picks up the background and ambient sounds that introduce errors into the precompensator adjustment procedure, especially in mobile applications where background noise is relatively high. The multipath echoes (or ring arounds) due to the acoustic environment are also a potential problem.
U.S. Pat. No. 6,408,079, entitled “Distortion Removal Apparatus, Method for Determining Coefficient for the Same, and Processing Speaker System, Multi-Processor, and Amplifier Including the Same”, issued Jun. 18, 2002 describes a frame-based precompensator based on the second order Volterra inverse of a second order Volterra model of the loudspeaker in the frequency domain. The precompensator uses the Volterra model in the forward direction and the Volterra inverse for precompensation. In one embodiment, the predistortion filter uses the Volterra expansion to model the loudspeaker in the forward direction and then an invertible exact inverse for precompensation.